Modern aircraft pilots must send and receive information to and from a large number of facilities. In particular, VHF radio signals are used by aircraft pilots for navigation. For example, VOR's (VHF Omni-directional ranges), localizers, LDA's, and SDF's are VHF radio signals that operate within the 108.00 to 117.95 MHz frequency band and have a power output necessary to provide coverage within an assigned operational service volume. The VHF navigation facilities that generate the VHF radio signals are subject to line-of-sight restrictions, and the range varies proportionally to the altitude of the receiving equipment.
Aircraft pilots are instructed to listen to the Morse Code identification of radio navigation stations to verify the correct VHF navigation facility has been tuned and that it is sending valid information. Most VOR facilities are equipped for voice transmission on the VOR frequency. VOR facilities without voice capability are indicated on navigation charts by an underline of the facility frequency. The voice capability may be used for two way communication with aircraft, for transmission of recorded flight information and for voice identification of the navigation station. Many Flight Service Stations (FSS) (or approach control facility) remotely operate several omniranges with different names. In some cases, none of the VOR facilities have the name of the “parent” FSS. Therefore, reliance on voice transmissions by the FSS involved for determining the identification of an omnirange is risky. During periods of maintenance, the facility may radiate a T-E-S-T code (- • •••-) or the code may be removed. The only positive method of identifying a VHF navigation facility is by decoding its Morse code identifier or monitoring the voice identification.
During operation of the aircraft, the pilot must identify the VHF navigation facility by mentally decoding an audio Morse code signal transmitted by the facility. Many pilots are not adept at performing this decoding operation, which often has to be performed during a high workload phase of flight. A single check, by the pilot, of the code at first use of the facility is not sufficient to detect a problem later in the flight. Continuos monitoring is necessary for the highest level of safety.
Systems that provide a radio Morse code identification function are in use and display the identification information in monochrome displays. While useful, such systems have limited functionality. Known Morse code identification systems typically sample the VHF navigation identifier at least twice to increase confidence of a correct decode. This increases the time before the identifier may be displayed to the pilot. Also, known Morse code identification systems do not correlate the decoded Morse code identifier with a specific VHF navigation facility, instead known systems only display a visual representation of the Morse code itself. When unable to decode the Morse signal, existing systems merely “dash” the display. These limitations increase the pilot's workload, and detract from safe aircraft operation and navigation.